Conventionally, a power transistor such as a metal-oxide semiconductor field effect transistor (MOSFET) is used in a DC/DC converter or the like.
FIG. 9 illustrates a configuration example of an existing DC/DC converter using a power transistor. A DC/DC converter 10 illustrated in FIG. 9 is a step-down type switching power circuit (synchronous regulator) that generates an output voltage Vout from an input voltage Vin and outputs the output voltage Vout.
The DC/DC converter 10 includes a transistor Q1 as a p-channel MOSFET power transistor, a transistor Q2 as an n-channel MOSFET power transistor, an inductor L1, a condenser C1, pre-drivers PD1 and PD2, and control logic units CL1 and CL2. These components constituting the DC/DC converter 10 are integrated in a single IC chip (power supply chip) to form a semiconductor device.
An upper transistor Q1 and a lower transistor Q2 are connected in series between an application terminal of an input voltage Vin and a ground terminal to from a bridge (switch output terminal). Specifically, a source of the transistor Q1 is connected to the application terminal of the input voltage Vin, a drain of the transistor Q1 is connected to a drain of the transistor Q2, and a source of the transistor Q2 is connected to a ground terminal.
A connection point between the drains of the transistors Q1 and Q2 is connected to one end of the inductor L1. The other end of the inductor L1 is connected to one end of the condenser C1 together with an output terminal of the output voltage Vout. The other end of the condenser C1 is connected to the ground terminal.
The pre-driver PD1 for outputting a driving signal to a gate of the transistor Q1 includes a transistor M11 as a p-channel MOSFET and a transistor M12 as an n-channel MOSFET. The upper transistor M11 and the lower transistor M12 are connected in series between an application terminal of a source voltage Vdd and the ground terminal to form a bridge.
The pre-driver PD2 for outputting a driving signal to a gate of the transistor Q2 includes a transistor M21 as a p-channel MOSFET and a transistor M22 as an n-channel MOSFET. The upper transistor M21 and the lower transistor M22 are connected in series between the application terminal of the source voltage Vdd and the ground terminal to form a bridge.
As the transistors M11 and M12 are complementarily (exclusively) turned on and off by the control logic unit CL1, the driving signal is generated by the pre-driver PD1 and output to the gate of the transistor Q1. Further, as the transistors M21 and M22 are complementarily (exclusively) turned on and off by the control logic unit CL2, the driving signal is generated by the pre-driver PD2 and output to the gate of the transistor Q2. The transistors Q1 and Q2 are complementarily (exclusively) turned on and off by the respective driving signals from the pre-drivers PD1 and PD2. Accordingly, a pulse-shaped switch voltage Vsw is generated from the input voltage Vin.
The term “complementarily (exclusively)” used herein includes a case in which a simultaneous OFF period of two transistors is set in terms of prevention of a through current, as well as a case in which ON/OFF of two transistors are completely reversed.
Further, the inductor L1 and the condenser C1 serves as a rectifying/smoothing circuit for rectifying/smoothing the switch voltage Vsw to generate a desired output voltage Vout.
Here, a schematic top view of a related-art transistor Q1 is illustrated in FIG. 10. Further, FIG. 10 illustrates a connection relationship of the transistor Q1 with the pre-driver PD1. As illustrated in FIG. 10, the transistor Q1 includes a comb-shaped gate electrode 11 including a plurality of extending portions 11A extending in an X direction and arranged in a Y direction substantially perpendicular to the X direction. Although not shown in FIG. 10, drain regions and source regions are alternately disposed in the Y direction between adjacent extending portions 11A. Further, the number of the extending portions 11A illustrated in FIG. 10 is set to 10 for the convenience of illustration, but actually it may be, for example, about 100.
Also, as illustrated in FIG. 10, a drain of the upper transistor M11 and a drain of the lower transistor M12, which constitute the pre-driver PD1, are connected to the gate electrode 11 by a wiring 21 formed of a metal. The driving signal is transmitted to the gate electrode 11 through the wiring 21.
Further, examples of the power transistor are disclosed in the related art.
As illustrated in FIG. 10, in order to secure a large amount of current in the transistor Q1 as the power transistor (i.e., in order to reduce ON resistance), the extending portions 11A of the gate electrode 11 extends in the extending direction (X direction) in the structure of the transistor Q1. Further, when the gate electrode 11 is formed of a material (polysilicon or the like) other than a metal, a circuit diagram of the transistor Q1 may be shown as illustrated in FIG. 11.
The transistor Q1 illustrated in FIG. 11 has a configuration in which transistors Q1-1 to Q1-10 corresponding to regions divided in the X direction of FIG. 10 are connected in parallel. Resistors R1 to R10 are connected in series in a line connected to the gates of the transistors Q1-1 to Q1-10.
According to this configuration, a charge amount of the driving signal per unit time is reduced in a direction from the resistor R1 to the resistor R10. Thus, for example, as illustrated in FIG. 12, a rise delay of a gate voltage is increased in a direction from the transistor Q1-1 to the transistor Q1-10 according to a gate capacitance of each transistor (in FIG. 12, only Q1-5 is illustrated as transistors between the transistors Q1-1 to Q1-10 for convenience). In other words, the response delay of the gate voltage is most significantly generated at the right end of the transistor Q1 of FIG. 10.
Thus, in a case in which the extending portions of the gate electrode formed of a material other than a metal are elongated, it is difficult to switch the transistor quickly. This is the same for the transistor Q2 as an n-channel MOSFET, as well as for the transistor Q1 as a p-channel MOSFET.